What green oak is
Green oak is freshly felled timber that has not yet dried. When a tree is felled, its cells are full of water — bound to the cell wall structure and free in the cell cavities. The moisture content of freshly felled oak is typically 60–100% of the timber’s oven-dry weight. At that moisture content, the timber is heavier, softer, and more workable than it will be once dry. It is also still living, in a biological sense: the moisture will move out of the timber over months and years until it reaches equilibrium with the surrounding environment.
The term “green” refers to its state, not its colour. Freshly cut oak heartwood is pale gold with a greenish cast that fades quickly on exposure to air. The green designation matters in construction because green oak behaves fundamentally differently from seasoned timber, and those differences determine how it can and cannot be used.
Why green oak is used for traditional framing
Traditional British timber frame construction — the method used for manor houses, barns, and churches from the medieval period until the late eighteenth century — was always done in green oak. The primary reason is the joint. A pegged mortise-and-tenon joint cut in green oak, assembled and pegged, tightens as both members dry. The mortise shrinks around the tenon. The seasoned oak peg locks the tenon in position and retains its dimensions while the surrounding green wood contracts. The result is a joint under permanent compression — a joint that is at its best state several years after assembly, not at the moment it was cut.
This behaviour is not achievable with seasoned timber. If you cut a mortise-and-tenon in seasoned oak at 10% moisture content and assemble it, the timber has already done most of its drying movement. The joint is stable but it is not under the same compression as a joint that was assembled green and dried in place. Traditional framing methods were developed around this material behaviour. Working in green oak is not a nostalgic choice — it is using the right material for the structural method.
What happens as green oak dries
As the moisture leaves the timber, the wood cells shrink. This shrinkage is not uniform in all directions: timber shrinks most across the grain (tangentially and radially) and very little along the grain. The differential shrinkage across the width and depth of a large-section member creates internal stresses. These stresses relieve themselves through checking — longitudinal splits that run with the grain.
Checking is normal. It is the timber’s response to drying, and it will happen in any large-section green oak member. The splits run along the grain and do not cross the grain in structural members — they do not reduce the beam’s ability to carry load in bending because they relieve stress rather than creating new failure planes. A beam that checks has not been damaged. A beam that does not check probably was not green when it was cut.
Large-section members — posts of 175mm square or more, tie beams of 200×150mm or larger — will develop more visible checks than small-section members. The checks are typically widest at the surface and taper as they go deeper. They are cosmetic, not structural. We explain this in writing at consultation stage, in the handover documentation, and on request at any point during the commission.
The frame will also twist slightly — posts may rotate a few millimetres at the top — and surface colour will change. The pale gold-green of fresh oak moves to silver-grey on exposed outdoor faces and to a warm golden-brown on sheltered indoor faces. These changes are predictable and understood. They are part of the material, not departures from it.
What clients should expect in the first year
The first winter is when most of the significant movement happens. The central heating season drops interior humidity; the frame dries faster than it would in an unheated space. Checking sounds — a quiet crack from somewhere in the frame — are common and sometimes alarming to clients who were not expecting them. They are the sound of the timber accommodating stress. They are not a structural event.
By the end of the first winter, most of the large checks will have opened and stabilised. The frame will continue to dry for several years, but subsequent changes are incremental rather than dramatic. After three to five years, the moisture content of the frame will have reached equilibrium with its environment and movement will effectively cease.
Clients should not attempt to fill checks in the first year. Any filler applied during the active drying period will be pushed out as the timber continues to move. If the aesthetic of the checks is a concern, they can be filled after the frame has reached equilibrium — typically after two to three years. We recommend natural wax or appropriate flexible filler for this purpose.
Seasoned timber
Seasoned timber has been dried — either by air drying over one to three years or by kiln drying to a target moisture content. our kiln brings timber to 8–15% moisture content depending on the application. At this moisture content, the timber has completed most of its drying movement. It is dimensionally stable: it will not produce checking or significant distortion when used in a building at typical indoor humidity levels.
Seasoned timber is the correct material for precision joinery work. A window frame must hold its dimensions to within a millimetre or two to seal correctly against draughts and driving rain. If the timber moves after installation, the seals fail, the draught bar cannot make contact, and the opening light binds in the frame. Seasoned timber, dimensionally stable, does not do this. Green timber in a window frame would be a serious error — the movement it undergoes as it dries would destroy the fit of the opening light within months.
When to use each
Green oak: traditional exposed structural frames — extensions, porches, garden rooms, barn repair work — where movement is acceptable, pegged joint construction is appropriate, and visual character matters. The material, the method, and the aesthetic are aligned. Using green oak in the wrong context produces problems; using it in the right context produces the best traditional timber frames available.
Seasoned timber: all joinery where dimensional stability is required — windows, doors, staircase components, door frames, fitted furniture. Also internal non-structural elements where movement would be visible or problematic: exposed floor boards, internal panelling, fitted shelving. In these applications, the premium for stability over green material is essential, not optional.
we use green oak for traditional framed structures and seasoned timber for all joinery and furniture. This is not a policy — it is the right material for each application, applied consistently across every project that comes through the workshop.